1. Field of the Invention
This invention relates to an apparatus for and a method of accelerating and pressurizing a fluidized stream of particulate matter for the purposes, for example, of duct transport over long distances and for the discharge of the fluidized streams at high velocities.
2. Description of the Related Art
In abrasive blast cleaning, such as with sand, grit or shot particles, velocity is imparted to particles which are directed against a surface to be cleaned, depainted, radioactively decontaminated or otherwise modified. The dynamic particle energy is converted into destructive forces which mechanically abrade or deform surface coatings. This methodology results in residual particulate matter of the blast stream, blast medium and the material removed as the blasting strips off the coating of the target surface, creating a high dust environment that may be hazardous to health, equipment and surrounding property. The cost of removing such matter may be excessive as well.
In addition, these blast particles are destructive when used for the treatment of fragile surfaces such as thin sheets, carbon and plastic.
Recently, less aggressive particulate matter such as dry ice and water ice has been utilized as blast particulate matter to avoid these problems, but not without limitations relating to transport and discharge. First, ice is not free flowing and must be "fluidized" with a gas, liquified gas or liquid in order to be transported to the target surface. Second, ice is not effective if discharged at low velocities. Third, ice is friable and heat sensitive and high velocity transport will generate considerable friction and heat and cause melting and breakdown of the ice particles. That said, the aim has been to achieve low transport and high discharge velocities within an apparatus that can handle all practical and useful types and sizes of particulate matter, including ice particles, and to control the sizing of particulate matter.
Previous practice of transporting or discharging fluidized particulate matter at high pressures, high velocities or both has involved the use of costly mechanical positive displacement pumps, which are volume dependent, complicated and do not mix or disperse or accelerate a fluidized stream well. Blowers, fans, and air jet and liquid jet pumps have also been used, but are only capable of generating small pressure increases and low velocities.
The use of single venturi nozzles as described in U.S. Pat. Nos. 4,038,786 and 4,707,951, in "Foundations of Aerodynamics" (A. M. Kuethe and J. D. Schetzer) and the "Mechanical Engineers' Handbook" (T. Baumeister and L. S. Marks) is ineffective for increasing pressure as can be achieved by induced flow created by injectors using either gas or liquid. Single venturi nozzles create increased velocity by gas expansion through falling pressures.
Amplifiers, such as taught by U.S. Pat. No. 4,389,820, have been used with limited success to induce flow in significant volumes, but unfortunately are able to generate only minimal pressure differentials and small increases in velocity. This is due to several inherent problems. First, the induction effect is dependent upon the boundary layer formation of a very thin high speed air film which is destroyed by the bombardment of particulate matter. Second, since the induction is via boundary layer shear viscous forces, there is minimal mixing and therefore little energy transfer to the bulk of the induced stream. Third, acceleration by usage of conduit restrictions will greatly affect or destroy the inductive effect, thereby placing a limitation on the effective increase in velocity that may be achieved. Fourth, air amplifiers, as the name implies, use a small amount of high velocity air to form a boundary layer to induce flow of a much larger amount of air and therefore there is little energy available to be transferred either for pressure or velocity increase. Finally, the foregoing limitations in mixing, velocity, available energy and pressure all preclude the possibility for effective high velocity discharge.
Oblique injectors of the form utilized in U.S. Pat. Nos. 4,555,872 and 5,203,794, where air or liquid is introduced via an opening in a main conduit after or before the entry of a particulate stream into the main conduit, have the chief advantage of providing for maximal turbulence and good mixing. However, these effects disturb the natural flow pattern of any incoming particulate stream, thereby preventing the possibility of forming an efficient nozzle. Because of this loss of efficiency, more energy and significant expense are required to achieve optimal pressures and velocities. The disturbance of the natural flow also results in regions of different velocities, thereby causing particulate deposition and plugging, erosion in the apparatus, and unwanted damage to friable, delicate particles including excessive size reduction.
As a variation of these injectors, gas or liquid injectors embodied within nozzles that extend into the main conduit thereby creating a multi-nozzle system have been practised in the art (U.S. Pat. Nos. 998,762, 4,806,171, and 4,817,342). In terms of discharge effectiveness, these systems use inefficient non-venturi converging nozzles, which release an uncontrolled expanded blast pattern. This pattern tends to concentrate the bulk of the particulate matter in a central region and consequently are not suitable for targeting large blast areas. The same may be said of component attachments such as are described in U.S. Pat. No. 4,843,770, which attempt to create a wider blast area using an uncontrolled expanded blast pattern. In addition, these systems tend to plug easily due to the use of non-fluid path defining nozzle body profiles, which create regions of different velocities and depositions.
In the U.S. Pat. No. 998,762, there is disclosed an apparatus for combining comminuted solids and liquids in which an internally rifled air nozzle discharges an air jet into a stream of solid particles, which then passes through a further nozzle. Both of the nozzles comprise a passage converging to an outlet mouth, so that the flow beyond the outlet mouths of the nozzles is uncontrolled. Consequently, the flow beyond the nozzle mouths is allowed to expand freely, to undergo turbulence and to produce excessive mixing, all of which will consume energy that could otherwise be directed for other purposes, and in particular for the acceleration of the solids.